The invention relates to a charge pump circuit, and in particular to a charge pump circuit for use in a phase locked loop frequency synthesizer.
Phase locked loops (PLLs) are used in wide variety of applications. Some primary uses involve data recovery, clock recovery from data streams in communication systems, and in frequency synthesizers which generally operate to use a lower frequency and synthesize a higher frequency.
Typically, a PLL frequency synthesizer will have a phase detector, a charge pump circuit, a loop filter and a voltage-controlled oscillator (VCO). The phase detector detects differences in phase between an input reference signal and a feedback signal from the VCO. The charge pump circuit generates either a positive current source (current source of fixed value I.sub.up) to add charge to the loop filter or a negative current source (current sink of fixed value I.sub.dn) to remove charge from the loop filter based on the error signal output from the phase detector. A third state does not change the current to the loop filter and instead provides a high impedance. The VCO uses the control voltage across the loop filter to minimize the frequency difference between the PLL feedback signal and the input reference signal.
The fundamental performance required by the PLL frequency synthesizer will depend on the performance of the charge pump circuit. The areas of critical importance include matching or good balance between the up and down currents (I.sub.up and I.sub.dn), which directly relates to reference sideband performance. The operating compliance of the circuit is important in that the charge pump circuit needs to operate over a wide voltage range, while maintaining the balance of the up and down currents.
Another important aspect is that the function must be implemented efficiently in terms of the quiescent supply current, being particularly true for low power designs. A final important aspect is switching speeds, which in a charge pump circuit is critical because of the need to switch the currents on and off at high speeds.